Plate heat exchanger

ABSTRACT

The invention relates to a plate heat exchanger ( 1 ) comprising at least one cassette ( 2   a - c ) formed by a pair of adjacent plates ( 3   a , b) in a plate stack ( 10 ). An inner heat transfer duct ( 4 ) runs between adjacent plates ( 3   a , b) within the cassette ( 2   a - c ). An insulating duct ( 6, 12 ) is disposed adjacent to the inner heat transfer duct ( 4 ) along at least one edge portion of the cassette ( 2   a - c ). The insulating duct ( 6, 12 ) is configured to insulate and separate the inside of the plate heat exchanger&#39;s heat transfer ducts ( 4, 11 ) from the environment outside the plate heat exchanger ( 1 ) in a direction transverse to a main heat transfer direction ( 7 ) through the plates ( 3   a, b ) of the cassette ( 2   a - c ) and thereby reduce temperature losses from the plate heat exchanger ( 1 ) to the surrounding environment.

FIELD OF THE INVENTION

The present invention relates to a plate heat exchanger for reducing temperature losses to the surrounding environment.

BACKGROUND TO THE INVENTION

Prior art endeavours to ensure that heat transfer between media in a heat exchanger reaches as high efficiency as possible. It is known that by placing a casing of insulating material round it the efficiency of a heat exchanger can be increased in that temperature losses to the surrounding environment outside the heat exchanger can thereby be reduced by the insulating extra material. A problem with applying such a casing is that it renders the manufacturing process more expensive and that the casing is subject to wear with the result that temperature losses may occur on portions of the heat exchanger where the insulating material becomes worn or damaged.

SUMMARY OF THE INVENTION

An object of the present invention is to create a plate heat exchanger with low temperature losses to the surrounding environment outside the plate heat exchanger. A further object of the invention is to provide a plate heat exchanger which is cost-effective compared with traditional technology and is easy to construct, enabling optimisation of cost and time.

The aforesaid and other objects are achieved according to the invention by the plate heat exchanger described in the introduction being provided with the features indicated in claim 1.

An advantage achieved with a plate heat exchanger according to claim 1 is that no external material round a plate stack in the plate heat exchanger is needed to reduce temperature losses to the surrounding environment.

Preferred embodiments of the plate heat exchanger according to the invention are further provided with the features indicated in subclaims 2-15.

According to an embodiment of the invention, each plate has an inner side and an outer side, the plates are laid on one another in a plate stack and every alternate plate in the plate stack is inverted, whereby the inner sides of two adjacent plates face towards one another, forming said cassette, and/or the outer sides of two adjacent plates face towards one another, forming between them an outer heat transfer duct. It is thus possible to use a single type of plate for constructing the plate stack in the plate heat exchanger. Manufacturing costs can thereby be reduced, since the number of tools and presses involved in making plates can be reduced.

According to an embodiment of the invention, the insulating duct is configured transversely to the direction of heat transfer in order to separate the outer heat transfer duct from the environment outside the plate heat exchanger. The inner heat transfer duct is configured to have a first medium flowing through it. The outer heat transfer duct created outside the cassette, between the cassette and an abutting third plate, is configured to have a second medium flowing through it. Heat transfer between the two media thus takes place through the respective plate in the cassette.

The direction of heat transfer means the main direction in which heat transfer takes place through a plate heat exchanger with stacked plates. The main heat transfer thus takes place in a direction substantially perpendicular to the respective heat transfer surfaces of the plates.

According to a further embodiment of the invention, the plates in the plate stack form adjacent cassettes whereby the outer heat transfer duct is disposed between the cassettes. The cassettes are formed in the plate stack by alternate plates in the stack each being inverted relative to an adjacent plate. The result is that the plate stack comprises a number of cassettes laid on one another.

According to a further embodiment of the invention, a second insulating duct is disposed along a second edge portion opposite to the first edge portion, whereby the inner heat transfer duct is situated between the first and second edge portions. The insulating duct and the inner heat transfer duct communicate with first and second port recesses. The first port is configured to receive an inflow of one medium and lead it into the cassette both through the inner heat transfer duct and through the insulating ducts. The port recesses are configured to communicate with a pair of corresponding port recesses of an adjacent cassette.

According to a further embodiment of the invention, a third and an opposite fourth insulating duct are disposed along a third and an opposite fourth edge portion of the cassette along the periphery of the cassette, said insulating ducts being so disposed that they together surround the inner heat transfer duct and form a continuous/connected insulating duct which extends round said periphery. The result is that the outer portion of the plate heat exchanger transversely to the direction of heat transfer insulates round the whole plate heat exchanger, whereby the inside of the plate heat exchanger is insulated from the environment on the outside with regard to reduction of temperature losses to said environment.

According to a further embodiment of the invention, each of said plates forming a cassette comprises: said edge portions; port recesses; the plate inner side with an inner heat transfer surface with an inner pattern; the plate outer side with an outer heat transfer surface with an outer pattern; thus the plate's inner pattern is disposed between a neutral plane which runs in the plate perpendicular to the direction of heat transfer through the plate and an inner plane which runs parallel in the plate's neutral plane, the plate's outer pattern is disposed between the neutral plane and an outer plane which runs parallel in the plate, and the height of the inner pattern between the inner plane and the neutral plane is less than the height of the outer pattern between the neutral plane and the outer plane. This means that the inner pattern extends on one side of the plate in one direction from the neutral plane, and the outer pattern extends on the other side of the same plate in an opposite direction from the neutral plane. The plates are configured to form respective cassettes in the plate heat exchanger and take the form of a single original type of plate.

According to a further embodiment of the invention, an outer sealing surface is disposed in the outer plane in the cassette's respective plates along the edge portions of the respective outer sides of the plates of the cassette on the opposite side from the region of the insulating ducts. An inner sealing surface is disposed in the inner plane in the cassette's respective plates along the edge portions on the inside of the cassette between each plate's adjacent inner side transversely to the direction of heat transfer outside the insulating ducts. The inner sealing surface in each plate is disposed transversely to the direction of heat transfer outside the outer sealing surface on the respective plate's edge portion. The respective plates in a pair of plates which form a cassette are connected to one another via the respective adjacent plate's inner sealing surface. The fact that the inner sealing surfaces of two adjacent plates are butted against one another and are connected creates a wall portion for the insulating ducts which is also the plate heat exchanger's outer side, which side is substantially parallel with the direction of heat transfer. The fact that the two adjacent outer sealing surfaces are butted against one another results in the respective insulating ducts therefore being adjacent to one another, forming round the plate heat exchanger an insulating layer which extends transversely to the direction of heat transfer and reduces temperature losses from the plate heat exchanger to the environment outside the plate heat exchanger.

According to a further embodiment of the invention, two adjacent cassettes are connected to one another via the respective plates' outer sealing surfaces, the outer heat transfer duct being disposed between the cassettes and delineated between the outer sealing surfaces. The inner heat transfer duct is adjacent to the outer heat transfer duct and the respective insulating ducts are adjacent to one another. As previously mentioned, the plate heat exchanger's respective inner heat transfer ducts and insulating ducts communicate with one another via respective first pairs of port recesses in the respective cassettes. The respective outer heat transfer ducts created between the cassettes communicate with one another via a second pair of port recesses in the respective cassettes. A variant of these second pairs of port recesses is that they are instead constituted by insulating ducts at two opposite edge portions being made somewhat smaller in their width parallel with the direction of heat transfer. This makes it possible for one medium to flow instead into the respective outer heat transfer ducts and through the plate heat exchanger via opposite edge portions, since the width of the insulating ducts is reduced, resulting in an aperture for throughflow between the cassettes.

According to a further embodiment of the invention, an end plate is placed against the plate stack's respective outermost plate, thereby providing an end duct between the plate and the end plate, the end duct communicates with at least one insulating duct in the plate heat exchanger, and the end duct is configured to insulatingly separate the inside of the plate heat exchanger from the environment outside the plate heat exchanger in the plate heat exchanger's direction of heat transfer. The fact that an end plate is placed against a first/last plate in the plate stack also stiffens the plate heat exchanger and further makes it possible for inward and outward pipe elements to be connected to at least one end plate in order to lead a medium into and out from the plate heat exchanger.

According to a further embodiment of the invention, adjacent plates in the plate heat exchanger are permanently connected to one another. The plates being permanently connected to one another results in a pressure-tolerant plate heat exchanger which is not temperature-sensitive. The plates may be connected to one another by, for example, soldering, welding, adhesive bonding etc.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the device according to the invention is described in more detail below with reference to the attached schematic drawings, which only show the parts necessary for understanding the invention.

FIG. 1 depicts a view of a portion of a plate heat exchanger in a section through the plate heat exchanger.

FIG. 2 a depicts one side of a plate and FIG. 2 b depicts the other side of the same plate.

FIG. 3 depicts a section through a plate heat exchanger in which the plates are separated from one another.

FIG. 4 depicts two adjacent plates separated from one another by an intervening plane.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

FIG. 1 depicts part of a plate heat exchanger (1) in a section through it, which plate heat exchanger (1) comprises a number of cassettes (2 a-c) stacked on one another in a plate stack (10). Upper and lower end plates (28 a, b) are disposed against the uppermost and lowest plates (3 a, b) of the plate stack (10). Within the plate stack (10), on the inside (30) of the plate heat exchanger (1), adjacent plates (3 a, b) form the following between them: insulating ducts (6, 12, 14, 15, see FIGS. 2 a, b), inner heat transfer ducts (4), outer heat transfer ducts (11), end ducts (29 a, b) between the respective end plates (28 a, b) and the uppermost and lowest plates (3 a, b) of the plate stack (10).

The respective plates (3 a, b) in the plate stack (10) derive from a single type of plate. To facilitate the stacking of the plates (3 a, b) against one another during the manufacturing process, they may in their respective adjacent edge portions (5, 13, 16, 17, see FIGS. 2 a, b) be provided with male/female embossing in the edge portions (5, 13, 16, 17). The statement that all of the plates (3 a, b), other than end plates, in said plate heat exchanger (1) are of a single type means in this specification that the plates (3 a, b) in said plate stack (10) are alike with regard to port recesses (18 a-d), to heat transfer surface (19, 21, see FIGS. 2 a, b), to patterns (20, 22, see FIGS. 2 a, b) in the heat transfer surface (19, 21) and to edge portions (5, 13, 16, 17) without regard to any possible embossings as above, and with regard to the configuration of insulating ducts (6, 12, 14, 15).

Said port recesses (18 a-d) as depicted in FIG. 1, FIG. 2 a and FIG. 2 b are illustrated in one size. It should be noted that this size may be varied and adapted according to the type of application in which the plate heat exchanger (1) is to be used. For example, if one medium takes the form of gas and the other medium the form of a liquid, one port pair, or at least one port, may for example have a larger diameter than the other ports. This is not depicted, however, in the drawings.

FIG. 2 a and FIG. 2 b both depict the same plate (3 a, b), FIG. 2 a representing an inner side (8) of the plate (3 a, b) and FIG. 2 b the other side of the plate, referred to as the plate outer side (9). As mentioned above, an edge portion (5, 13, 16, 17) extends round the periphery of the plate (3 a, b). The edge portion (5, 13, 16, 17) is pressed in such a way that it comprises a number of different levels, also called planes (23-25, see FIG. 4), which planes are described in more detail further on herein in relation to FIG. 4. The plate inner side (8) in FIG. 2 a comprises on the edge portion (5, 13, 16, 17) an inner sealing surface (27) disposed furthest out on the edge portion (5, 13, 16, 17) along the periphery round the plate (3 a, b). The inner sealing surface (27) is configured for butting against a corresponding inner sealing surface (27) on an adjacent plate (3 a, b). An inner heat transfer surface (19) which comprises port recesses (18 a-d) and an inner pattern (20, see FIG. 4) is disposed within the edge portion (5, 13, 16, 17). The plate outer side (9) in FIG. 2 b comprises on the edge portion (5, 13, 16, 17) an outer sealing surface (26) disposed within the inner sealing surface (27) on the plate (3 a, b). The outer sealing surface (26) is disposed at a level which is different from the inner sealing surface (27). Said outer sealing surface (26) is configured to be butted against the outer side (9) of an adjacent plate (3 a, b). An outer heat transfer surface (21) comprising an outer pattern (22, see FIG. 4) is disposed on the plate outer side (9). The edge portion (5, 13, 16, 17) round the plate (3 a, b) is disposed in such a way that when two inner sides (8) of two adjacent plates (3 a, b) are butted against one another, the result in the edge portion (5, 13, 16, 17) between the plates (3 a, b) is an insulating duct (6, 12, 14, 15). This is illustrated in FIG. 2 a in that three inner portions of the edge portion (5, 13, 16, 17) which are shown bracketed together after the inner sealing surface (27) represent insulating ducts (6, 12, 14, 15) along the respective edge portions (5, 13, 16, 17). The patterns (20, 22) are configured both to increase the heat transfer surface and to create turbulence in a flow of a medium passing across and between two adjacent plates (3 a, b) in the plate heat exchanger (1). Port recesses (18 a-d) disposed on the plate inner side run through the plate (3 a, b) to the plate outer side (9).

The plates (3 a, b) according to FIGS. 2 a and 2 b in the plate stack (10) according to FIG. 1 are stacked on one another in such a way that every alternate plate (3 b) is inverted, whereby the inner side (8) of each plate (3 a) is adjacent to the corresponding inner side (8) of an adjacent plate (3 b), and an outer side (9) of a plate (3 a) is adjacent to the corresponding outer side (9) of an adjacent plate (3 b). A cassette (2 a-c) as above is formed by two adjacent plates (3 a, b), whereby the plates (3 a, b) of each cassette (2 a-c) have their inner sides (8) facing one another. In the plate stack (10), an inner heat transfer duct (4) runs inside each cassette (2 a-c) between the inner sides (8) of two adjacent plates (3 a, b). An outer heat transfer duct (11) runs between two adjacent cassettes (2 a-c). This corresponds to the outer heat transfer duct (11) running between the outer sides (9) of two adjacent plates (3 a, b).

The inner heat transfer duct (4) is configured to have a first medium flowing through it. This first medium also flows in the insulating ducts (6, 12, 14, 15) which communicate with the inner heat transfer duct (4). The outer heat transfer duct (11) is configured to have a second medium flowing through it. The media have between them a mutual heat exchange which takes place through the respective plate (3 a, b) as a result of one medium flowing on one side of the plate (3 a, b) and the other medium flowing on the other side of the same plate (3 a, b).

In a direction conforming to the heat transfer direction (7) through the plates (3 a, b) in the plate heat exchanger (1), the inner heat transfer duct (4) is adjacent to the outer heat transfer duct (11). Along the edge portions (5, 13, 16, 17) of the plate heat exchanger (1), the stack side of the plate heat exchanger (1) in the heat transfer direction (7) takes the form of adjacent insulating ducts (6, 12, 14, 15) laid on one another. A medium preferably at a lower temperature than the medium which flows in the outer heat transfer ducts (11) within the plate heat exchanger (1) flows in the insulating ducts (6, 12, 14, 15). The insulating ducts (6, 12, 14, 15) thus insulate the plate heat exchanger (1), thereby reducing its heat losses to the surrounding environment. According to a variant, the temperatures of the media may be the other way round, in which case the medium which flows in the inner heat transfer duct (4) and the insulating ducts (6, 12, 14, 15) will be warmer than the medium which flows in the outer heat transfer ducts (11).

As mentioned above, each plate (3 a, b) comprises port recesses (18 a-d) configured to serve as inlets and/or outlets for a flow of a medium through the plate heat exchanger (1). The port recesses (18 a-d) are disposed in the end plate (28 a) so that it can be connected to pipe elements (not depicted in the drawings) in order to lead said media into and out from the plate heat exchanger (1). According to a variant, at least one port recess (18 a-d) may also be disposed on the second end plate (28 b) of the plate heat exchanger (1).

FIG. 3 depicts a number of plates (3 a, b) in section through the plate heat exchanger (1) according to the invention, which plates (3 a, b) are separated from one another by intermediate spaces to clarify their construction and mutual relationships. The inner heat transfer duct (4) runs between the mutually facing inner sides (8) of two adjacent plates (3 a, b). An inner sealing surface (27) is disposed along the edge portion (5, 13, 16, 17, see FIGS. 2 and 4) round the periphery of the respective plates (3 a, b) on their inner sides (8). The adjacent inner sealing surfaces (27) are butted against one another and connected to one another, e.g. by soldering or by some other known connection technique. The insulating ducts (6, 12, 14, 15) are disposed between the inner sealing surface (27) and the inner heat transfer duct (4). The inner heat transfer duct (4) is surrounded by insulating ducts (6, 12, 14, 15) which extend round said inner heat transfer duct (4) in each cassette (2 a-c) (see FIGS. 2 a, 2 b). An outer sealing surface (26) is disposed on the outer sides (9) of the plates (3 a, b). This outer sealing surface (26) extends along the edge portions (5, 13, 16, 17) of the plate (3 a, b) on the latter's outer side (9) on the opposite side from where insulating ducts (6, 12, 14, 15) are disposed on the plate inner side (8). The inner sealing surface (27) is configured for butting against the outer side (9) of an adjacent plate (3 a, b), which outer side (9) comprises a corresponding inner sealing surface (27). Butting two outer sealing surfaces (26) on two opposite plates (3 a, b) against one another creates the outer heat transfer duct (11) inside this sealing surface (26), within the plate heat exchanger (1). FIG. 3 depicts the uppermost and lowest end plates (28 a, b) which are configured for butting against the uppermost and lowest plates (3 a, b) in the plate stack (10). A duct, referred to as an end duct (29 a, b), is thus created between the uppermost and lowest plates (3 a, b) and the respective end plate. This end duct (29 a, b) on each side of the plate heat exchanger (1) has the function, in the same way as the insulating ducts (6, 12, 14, 15), of serving as an insulating layer for preventing temperature losses to the surroundings outside the plate heat exchanger (1) in the heat transfer direction (7) through the plates (3 a, b).

FIG. 4 depicts two plates (3 a, b) facing one another with levels for planes (23-25) illustrated to clarify their positions and mutual relationships. In FIG. 4, the upper plate (3 a) has its inner side (8, see FIG. 2 a) facing downwards into the drawing and its outer side (9, see FIG. 2 b) facing upwards. The lower plate (3 b) has its inner side (8) facing upwards towards the upper plate (3 a) and its outer side (9) facing downwards.

The upper plate (3 a) comprises an inner plane (24), an outer plane (25) and a neutral plane (23) which is disposed between the planes (24, 25). In the upper plate (3 a) in FIG. 4, the inner plane (24) is disposed below the neutral plane (23) and the outer plane (25) is disposed above the neutral plane (23). The upper plate (3 a) has a first edge portion (5) and a second edge portion (13) which is opposite to the first edge portion (5). Between the edge portions (5, 13), an inner pattern (20) is disposed on the inner side (8) of the plate (3 a), and an outer pattern (22) is disposed on the outer side (9) of the plate (3 a). FIG. 4 illustrates only how the edge portions designated 5 and 13 relate to one another, but it should be noted that the previously mentioned opposite edge portions designated 16 and 17 (see FIGS. 2 a, b) correspond in position and function to the edge portions according to FIG. 4. The outer sealing surface (26) is disposed on the plate outer side (9) at the respective edge portions (5, 13). Insulating ducts (6, 12) and the inner sealing surface (27) are disposed on the plate inner side (8) at the respective edge portions (5, 13).

The inner sealing surface (27) is disposed in the inner plane (24) and the outer sealing surface (26) is disposed in the outer plane (25).

The lower plate (3 b) in FIG. 4 is a plate (3 b) according to the upper plate (3 a) but faces in such a way that its components and planes (23-25) mirror said components and planes (23-25) as above in the upper plate (3 a).

A cassette (2 a-c) is formed by the plates (3 a, b) according to FIG. 4 being butted against one another and connected to one another in the common inner plane (24) of the plates (3 a, b). The inner pattern (20) comprises in adjacent plates (3 a, b) small ridges which are butted against and connected to one another. The outer pattern (22) comprises large ridges which are butted against and connected to the large ridges of the outer pattern (22) on the outer side (9) of an adjacent plate (3 a, b).

The invention is not limited to the embodiment depicted but may be varied and modified within the scope of the claims set out below, as partly described above. 

1. A plate heat exchanger (1) comprising at least one cassette (2 a-c) formed by a pair of adjacent plates (3 a, b), whereby an inner heat transfer duct (4) and, adjacent thereto along at least an edge portion (5, 13, 16, 17) of the cassette (2 a-c), an insulating duct (6, 12, 14, 15) run between the plates (3 a, b) within the cassette (2 a-c), whereby insulating ducts (6, 12, 14, 15) are configured to separate the inner heat transfer duct (4) from the environment outside the plate heat exchanger (1) in a direction transverse to a main heat transfer direction (7) through the plates (3 a, b) of the cassette (2 a-c).
 2. A plate heat exchanger (1) according to claim 1, in which the respective plates (3 a, b) each comprise an inner side (8) and an outer side (9), which plates (3 a, b) are laid on one another in a plate stack (10), every alternate plate (3 a, b) in the plate stack (10) being inverted, whereby the inner sides (8) of two adjacent plates (3 a, b) face towards one another, forming said cassette (2 a-c), and/or the outer sides (9) of two adjacent plates (3 a, b) face towards one another, forming between them an outer heat transfer duct (11).
 3. A plate heat exchanger (1) according to claim 2, in which insulating ducts (6, 12, 14, 15) are configured transversely to the heat transfer direction (7) to separate the outer heat transfer duct (11) from the environment outside the plate heat exchanger (1).
 4. A plate heat exchanger (1) according to claim 2, in which the plates (3 a, b) in the plate stack (10) form adjacent cassettes (2 a-c), whereby the outer heat transfer duct (11) runs between the cassettes (2 a-c).
 5. A plate heat exchanger (1) according to claim 1, in which a second insulating duct (12) is disposed along a second edge portion (13) opposite to the first edge portion (5), whereby the inner heat transfer duct (4) is disposed between the first and second edge portions (5, 13).
 6. A plate heat exchanger (1) according to claim 1, in which a third and an opposite fourth insulating duct (14, 15) are disposed along a third and an opposite fourth edge portion (16, 17) of the cassette (2 a-c) along the periphery of the cassette (2 a-c), and said insulating ducts (6, 12, 14, 15) are so disposed that they together surround the inner heat transfer duct (4) and form an insulating duct (6, 12, 14, 15) which extends round said periphery.
 7. A plate heat exchanger (1) according to claim 1, in which each said plate (3 a, b) forming a cassette (2 a-c) comprises: said edge portions (5, 13, 16, 17); port recesses (18); the plate inner side (8) with the inner heat transfer surface (19) with an inner pattern (20); the plate outer side (9) with an outer heat transfer surface (21) with an outer pattern (22); whereby the inner pattern (20) of the plate (3 a, b) is disposed between a neutral plane (23) which runs in the plate (3 a, b) perpendicular to the heat transfer direction (7) through the plate (3 a, b) and an inner plane (24) which runs parallel in the neutral plane (23) of the plate (3 a, b); the outer pattern (22) of the plate (3 a, b) is disposed between the neutral plane (23) and an outer plane (25) which runs parallel in the plate (3 a, b); and the height of the inner pattern (20) between the inner plane (24) and the neutral plane (23) is less than the height of the outer pattern (22) between the neutral plane (23) and the outer plane (25).
 8. A plate heat exchanger (1) according to claim 1, in which an outer sealing surface (26) is arranged in the outer plane (25) in each plate (3 a, b) of the cassette (2 a-c) along the edge portions (5, 13, 16, 17) of the respective outer sides (9) of the plates (3 a, b) of the cassette (2 a-c) on the opposite side from the region of the insulating ducts (6, 12, 14, 15).
 9. A plate heat exchanger (1) according to claim 1, in which an inner sealing surface (27) is disposed in the inner plane (24) in each plate (3 a, b) of the cassette (2 a-c) along the edge portions (5, 13, 16, 17) on the cassette (2 a-c) between the adjacent plate side (8) of the respective plate (3 a, b) transversely to the heat transfer direction (7) outside the insulating ducts (6, 12, 14, 15).
 10. A plate heat exchanger (1) according to claim 9, in which the inner sealing surface (27) in each plate (3 a, b) disposed transversely to the heat transfer direction (7) is outside the outer sealing surface (26) on the edge portion (5, 13, 16, 17) of the respective plate (3 a, b).
 11. A plate heat exchanger (1) according to claim 9, in which the respective plates (3 a, b) in a pair of plates forming a cassette (2 a-c) are connected to one another via the inner sealing surface (27) of the respective adjacent plate (3 a, b).
 12. A plate heat exchanger (1) according to claim 9, in which two adjacent cassettes (2 a-c) are connected to one another via the outer sealing surface (26) of the respective plate (3 a, b), whereby the outer heat transfer duct (11) runs between the cassettes (2 a-c) and is delineated between the outer sealing surfaces (26).
 13. A plate heat exchanger (1) according to claim 2, in which the inner heat transfer duct (4) is adjacent to the outer heat transfer duct (11), and the respective insulating ducts (6, 12, 14, 15) are adjacent to one another.
 14. A plate heat exchanger (1) according to claim 2, in which an end plate (28 a, b) is disposed against the outermost plate (3 a, b) of the plate stack (10), whereby an end duct (29 a, b) runs between the plate (3 a, b) and the end plate (28 a, b), the end duct (29 a, b) communicates with at least one insulating duct (6, 12, 14, 15) in the plate heat exchanger (1), and the end duct (29 a, b) is configured to insulatingly separate the inside (30) of the plate heat exchanger (1) from the environment outside the plate heat exchanger (1) in the heat transfer direction (7) of the plate heat exchanger (1).
 15. A plate heat exchanger (1) according to claim 1, in which adjacent plates (3 a, b) in the plate heat exchanger (1) are permanently connected to one another. 